This has indeed changed. I went on a test run with a driver some time ago, and he advised that he was (at that time) permitted to use his mobile when driving provided that he was running under green signals.
DBS has since modified its Policy, and drivers are no longer allowed to use mobile phones when the loco is moving. I am not sure whethher they are allowed to use them when staitionary though.
ASLEF have stickers that have been unofficially stuck in class 66 cabs, which say "Don't let your mobile cost you your job".
The EWS rule on mobile usage has developed over the years so to speak! The most recent form of the rule when I was at EWS/DB Schenker was that you were not to use your mobile whilst a train was in motion although you could receive and make calls at a booked calling point. Being held at a signal you were permitted to telephone the signaller (if no STP was available) and you were also permitted to telephone the Trust delay attribution team. I expect that the rule has changed further still though.